I am not sure of the exact reason, but they could have posted link to their products or posted saying "we are a company....". This could have been viewed as spam. If it wasn't spam, commercial sellers need approval from an Admin. Have a look at the snippet of the forum rules below.
My suggestion is for them to email one of the Admins and to clear it with them. Who knows, it could be the beginning of a commercial partnership.
For info here is the forum rules in regards to commercial sellers:
As Mp3car.com is a commercial enterprise, commercial advertisers are allowed only with the approval of the mp3car.com forum administrators. If you would like to request permission to advertise on these forums, please email one of the administrators. We have implemented Google Ads for unregistered members for this purpose. Links to other sites that sell comparable products to the mp3car.com store are acceptable to point out product specs or comparison pricing as an aid to other members. Acting as an agent for another website selling similar products and advertising the website or products on the forums is not permitted.
Spam is defined as unwanted, unsolicited commercial emails. While great lengths have been taken by the administration to prevent spam from appearing on the forums, it still happens. Members can help deal with spam by reporting the spam post. This will alert administrators & moderators so that they can take action.
Hope this helps.